I also had an error while installing grub "warning: your core.img is unusually large. It won't fit in the embedding area."

Almost certainly. GRUB 2 is very complex, and there's no way it will fit in the "traditional" areas for a boot loader on a BIOS-based computer -- namely, the 440 bytes reserved for boot loaders in the MBR and a small space at the start of the boot partition. Therefore, GRUB 2 takes over certain "unallocated" sectors on an MBR disk -- namely, whatever sectors exist between the MBR and the first partition on the disk. Using old-style "cylinder" alignment, this is normally 62 sectors (31 KiB), but on newer disks with 1MiB alignment, it's just under 1MiB. On GPT disks, GRUB 2 uses a dedicated partition, known as the BIOS Boot Partition, which can be whatever size you choose when you partition the disks, although 1MiB is a common size. So:

If you're using an MBR disk, you can try resizing your first partition so that it starts a bit later. Unfortunately, this can be time-consuming and dangerous if it's a big partition (like an LVM physical volume). If you've got swap space or a /boot partition you want to retire in that space, you should be able to do this without too much hassle.

If you're using a GPT disk, you can try creating a new and larger BIOS Boot Partition to replace the one you've got now. This may require resizing another partition, but you'll at least have more options, and you should be able to shrink a partition by shaving off a bit of space at its end, which is safer than moving the end of a partition. I recommend deleting your old BIOS Boot Partition to avoid confusion or accidental use of the wrong partition.